crypto: Clean up help text for CRYPTO_SHA256

crypto: Clean up help text for CRYPTO_SHA256

NFS, Ceph, SMB, and Btrfs stopped using this SHA-256 implementation in
commits c2c90a8b2620626c ("nfsd: use SHA-256 library API instead of
crypto_shash API"), 27c0a7b05d13a0dc ("libceph: Use HMAC-SHA256 library
instead of crypto_shash"), 924067ef183bd17f ("ksmbd: Use HMAC-SHA256
library for message signing and key generation"), and fe11ac191ce0ad91
("btrfs: switch to library APIs for checksums").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/bf8e1c229b36fc5349e29701e962d0dfd4fd21b6.1772116160.git.geert+renesas@glider.be
Signed-off-by: Eric Biggers <ebiggers@kernel.org>